Local Area Networks (LAN) such as Ethernet or token-ring networks, are generally interconnected through hubs. The hub is a system made of LAN adapters that communicate together through a switch card containing a switch engine. Such a switch engine can be either a shared memory switch or a crossbar switch.
The shared memory switch is a device wherein the data packets received by the input ports are stored into a memory at address locations determined by queues containing the packet destination addresses; the packets to be transmitted on the output ports as the destination addresses are de-queued.
Although such a switch enables the transfer of data packets it may present a bottleneck due to the hardware requirement of the many output ports, and may stop the data packet transfer from one adapter to another one due to hardware failure. In this particular case all data packets stored in the switch memory will be lost. This particular failure will have an impact on data transfer and on customer applications at the end of the adapter.
Any hardware failure on any switch system is damageable on the data packet transfer, and one of the solutions to prevent any hardware failure is to duplicate the switch system.
This solution consists of having duplicated synchronized switch cards, where one switch is active and the other one is backup. Both switches operate in parallel meaning that they receive/transmit the same data packets from/to the LAN adapters and receive the same commands issued from a Control Point of the switching system. When the active switch receives a switchover command from the Control Point, the active switch stops transferring data to the adapter. At the same time the backup switch receiving the same command becomes the active switch for all the attached adapters and takes over the data transfer. The major problem of those existing active/backup switches configurations is the loss or the duplication of data packets during the switchover operation, because the two switches are not in reality fully data packet synchronized.
Therefore there is a need for a switchover system that avoids loss or duplication of data packets during the switchover process.